1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that uses a plurality of laser beams to generate marks on a recording medium and that records the marks as an image.
2. Discussion of the Background
In general, many image forming apparatuses that record information on a recording medium using light, such as laser light, have used a single light beam and are therefore required to write an image in sequence on a recording medium with the single light beam. These single-beam image forming apparatuses are typically required to transfer an information signal for modulating the single beam at a relatively high speed in an event of printing an image at a relatively high speed or with a relatively high density. At the same time, the single-beam image forming apparatuses are required to perform a main scanning at a relatively high speed. Accordingly, those apparatuses which use a rotary polygon mirror for scanning the recording medium are required to drive the rotary polygon mirror at a relatively high rotational speed which may reach tens of thousands r.p.m. (revolutions per minute). Therefore, such a single-beam image forming apparatus has a structural limitation to perform a relatively high speed recording operation.
For the reasons as described above, an image forming apparatus which uses a plurality of light beams (multi-beams) has been developed. In such a multi-beam image forming apparatus, the light beams are modulated by each of individual recording signals and simultaneously scan a recording medium. FIG. 14 illustrates an exemplary disposition of light beams in such a multi-beam image forming apparatus that uses a four-laser-beam optical writing device. In this multi-beam optical writing device, distances between adjacent beams among four laser beams B1, B2, B3, and B4 are needed to be equal to a distance between adjacent pixels (hereinafter referred to as a pixel distance) to be formed on the recording medium.
On the other hand, a semi-conductor laser array (hereinafter referred to as a laser diode) that is popularly used as a light beam source for an optical writing device generally has a distance between light emission points (hereinafter referred to as a light emission point distance) which is not equal to, and which is generally greater than, a pixel distance required by a recording device which has a current mainstream resolution of 600 dpi (dots per inch), for example. In order to adjust this difference, as illustrated in FIG. 14, a disposition of four light emission points is inclined at a preset angle xcex81 relative to a line L-Lxe2x80x2 that is orthogonal to a beam scanning direction SL, so that actual beam distances between adjacent beams become equal to a pixel distance Ps.
In the above-described optical writing device, the laser beams B1 to B4 scan a beam detector including at least a photosensor. The beam detector outputs beam detection signals when detecting the laser beams B1 to B4. An exposure starting position (hereinafter referred to as an xe2x80x9cimage write start positionxe2x80x9d)on the recording medium for each of the laser beams B1 to B4 is determined on the basis of the beam detection signals from the beam detector.
Although various constructions of a beam detector are hitherto known, there are generally two types. One type of beam detector detects a plurality of laser beams with one detector (light detector for detecting the laser beams), and the other type of beam detector detects a plurality of laser beams using a same number of light detectors.
FIGS. 15 to 18 illustrate cases of using a light detector d which has a relatively long beam detecting surface. FIG. 15 illustrates a manner in which the beam detector d is positioned with an end face L-Lxe2x80x2 thereof made orthogonal to the beam scanning direction SL. In this case, if the operation for writing an image to the recording medium is started at a certain time after each of the laser beams B1 to B4 is received by the beam detector d, a vertical line of the image can be formed as a straight line that is parallel to the aforementioned end surface L-Lxe2x80x2.
Namely, when times t1, t2, t3, and t4 at which each of the respective laser beams B1 through B4 reaches the image write start position S passing through the end surface L-Lxe2x80x2 are set to a uniform time, the vertical line formed by performing a plurality of scanning operations can be configured to a straight line in parallel with the end surface L-Lxe2x80x2, as shown in FIG. 16.
As shown in FIG. 17, for example, in some cases the beam detector d may be mounted with the end surface L-Lxe2x80x2 slightly inclined (at an angle xcex82) relative to a line orthogonal to the beam scanning direction SL due to a mechanical inaccuracy. In such a case, when each of the laser beams B1 through B4 starts writing an image on the recording medium at a certain time after each of the laser beams B1 through B4 is detected by the beam detector d, as illustrated in FIG. 17, the line formed by connecting at least four image write start positions S is not orthogonal to the beam scanning direction SL. As a result, when the scanning operation is repeated, a vertical line becomes a jagged line having a cycle of four laser beams B1 through B4, as illustrated in FIG. 18.
In a case that the beam detector d includes a plurality of light detectors that form beam detectors d1, d2, d3, and d4 for detecting the laser beams B1 to B4, it is hard to vertically align the beam detectors d1, d2, d3, and d4 since a beam pitch distance is relatively narrow. Accordingly, the beam detectors d1, d2, d3, and d4 may be positioned in a slanting manner as illustrated in FIG. 19. If the beam detectors are disposed in such a manner, a straight line orthogonal to the beam scanning direction SL cannot be formed unless the time from detecting the beam by each of the beam detectors d1 through d4 to starting the operation of writing an image on the recording medium is individually set.
Namely, in such a situation it is required to adjust and set the timing to start writing images (hereinafter sometimes referred to as an xe2x80x9cimage write start timingxe2x80x9d). The image write start timing for the laser beam B1 is set to a time after a time period of t+t1 from a time when the beam detector d1 detects the laser beam B1. The image write start timing for the laser beam B2 is set to a time after a time period of t+t2 from a time when the beam detector d2 detects the laser beam B2. The image write start timing for the laser beam B2 is set to a time after a time period of t+t3 from a time when the beam detector d3 detects the laser beam B3. The image write start timing for the laser beam B2 is set to a time after a time period of t+t4 from a time when the beam detector d4 detects the laser beam B4.
An exemplary method is described in Japanese Laid-Open Patent Publication No. 57-8887/1982 of a control of the image write start position S for an optical writing in a recording device that uses a plurality of laser beams to scan a recording medium to record an image on the recording medium. However, this exemplary control method uses clock signals which have a frequency n times faster than that of pixel clock signals to control the number of pulses of the beam detection signal. Therefore, this method requires a counting of extremely high frequency pulses and, at the same time, a setting of the image write start timing for each of the laser beams individually. Accordingly, in this method, setting values of all the image write start timings for the laser beams are required to be readjusted, even at only a slight adjustment of an image position. This causes a problem for users in adjusting a position of an image on a recording sheet.
Therefore, it is believed that there is no image forming apparatus available that allows a user to perform an easy adjustment of an image position, including a delicate image positioning, without having a jagged line on an image.
In light of the above and other problems, an object of the present invention is to provide a novel image forming apparatus capable of adjusting an image write start position by adjusting the same at a time difference shorter than a cycle of a reference clock signal with ease.
According to an aspect of the present invention, a novel image forming apparatus of the present invention forms an image by scanning a recording medium with a plurality of light beams that are modulated by individual image forming signals respectively corresponding to the light beams. In this novel image forming apparatus a beam detecting device detects the plurality of light beams at preset positions and outputs corresponding detection signals. Further, at least one delay device delays each of the detection signals from the beam detecting device by an arbitrary time period which is predetermined individually for each of the detection signals. Moreover, a reference clock signal generating device generates a plurality of reference clock signals on the basis of delayed detection signals output from the at least one delay device. The image forming apparatus performs an image forming operation according to the plurality of reference clock signals.
At least one delay device of the image forming apparatus may delay one of the light beams detected at a selected one of the preset locations closest to a predetermined image write start position by a predetermined and fixed delay time.
In the novel image forming apparatus a number of delay devices may be the same as a number of the light beams.
In the novel image forming apparatus one delay device may be provided for delaying each of the plurality of light beams.
The novel image forming apparatus may further include at least one delay device that delays each of the detection signals from the beam detecting device by an arbitrary time period which is predetermined individually for each of the detection signals, and that controls the image write start position of each of the light beams.
The delay device of the image forming apparatus may further include a first delay device that delays all of the detection signals from the beam detecting device by a same time period and a second delay device that delays each of the detection signals from the beam detecting device by an arbitrary time period which is predetermined individually for each of the detection signals. Further, the delay device can control each of image write start positions of the light beams according to each of delayed detection signals output from the second delay device.
Scanning operations of the plurality of light beams of the novel image forming apparatus can be performed by a rotary polygon mirror and the arbitrary time period of the delay device can be automatically set in response to a change of a rotation speed of the rotary polygon mirror.